The present invention relates to a semiconductor laser which applies to and is suitable for optical communication and a photo module which uses the semiconductor laser.
A long-wavelength-band semiconductor laser which emits laser beam in a wavelength band of 1.3 xcexcm or more is a principal device of optical communication, and, at present, is chiefly constituted by forming a compound semiconductor layer made of a material, such as InGaAsP, InAlGaAs, or InAsP, on an InP substrate. Because the InP substrate is expensive and the substrate size is difficult to increase, however, the semiconductor laser which uses the InP substrate was forced to become expensive.
On the contrary, a GaAs substrate is comparatively inexpensive and the substrate size is easy to increase. There was a problem, however, that the material which can be formed on the GaAs substrate is limited in terms of lattice strain and a semiconductor laser of high practicality in a long wavelength band is difficult to obtain. Because the GaAs substrate has the aforementioned features, however, the research and development of the long-wavelength-band semiconductor laser which uses GaAs in a substrate is advancing powerfully. Besides, if a long-wavelength-band vertical cavity surface emitting laser which uses the GaAs substrate can be realized, the laser can be combined with a GaAs/AlAs semiconductor multiple layer mirror and enables more miniaturization and realization of lower cost.
GaInNAs, GaAsSb, and InAs quantum dots are accepted as active layer materials which can be fabricated on the GaAs substrate and emit laser beam in a long wavelength which exceeds 1.3 xcexcm. The lattice strain in each active layer material is approximately 2%, approximately 2.6%, and approximately 7% in order. Because the lattice strain (lattice-mismatch between the substrate and the active layer) is large as much as 2% or more in all active layer materials, such a problem that the life of a device is short may possibly occur.
On the other hand, there is an active layer which uses a type-II heterojunction structure as the active layer material which can be fabricated on the GaAs substrate in the same manner and emit laser beam in a long wavelength band which exceeds 1.3 xcexcm. In a type-I heterojunction structure adopted in many usual semiconductor lasers, a semiconductor layer which forms a quantum well in a conduction band forms the quantum well also in a valence band and emission occurs in the same material. On the contrary, in the type-II, as described later, the layer adjacent to the semiconductor layer which forms the quantum well in the conduction band forms the quantum well in the valence band and the emission occurs between different materials. In the active layer which uses the type-II heterojunction structure, there is such an advantage that the degree of design freedom of energy band structure and lattice strain is high.
A laser device which uses the type-II heterojunction structure based on GaAsSb/InGaAs grown epitaxially on the GaAs substrate in the active layer is disclosed in a US document xe2x80x9cJournal of Vacuum Science and Technologyxe2x80x9d B18 (published in 2000), on pages 1,605 to 1,608, for example. In this example, the active layer is fabricated using one layer of GaAsSb/InGaAs respectively.
Further, a laser device structure, in which a type-II superlattice constituted of the GaAsSb layer and the InGaAs layer which are thin as many as 1 to 10 molecular layers is used in the active layer, is disclosed in Japanese Patent Laid-open (Kokai) No. 2000-164990.
In the active layer which uses the type-II heterojunction structure, there is the aforementioned advantage that the degree of design freedom of the energy band structure is high, but there is the following problem and it was difficult to put the active layer to practical use.
FIG. 9 shows the energy band structure in an example of the aforementioned laser having the type-II heterojunction structure in which one layer of the GaAsSb layer and the InGaAs layer is used respectively. The horizontal axis of FIG. 9 shows a semiconductor layer which is grown in the right direction from a substrate in order. With respect to the vertical axis, the right-side axis which shows distribution of electrons (holes) is used for a wave function and the left-side axis which shows energy is used for another function. A bottom barrier layer 57a, an InGaAs layer 55, a GaAsSb layer 56, and a top barrier layer 57b are formed from the substrate in order, and the InGaAs layer 55 and the GaAsSb layer 56 become active layers.
In FIG. 9, the energy band structure is constituted of an energy of conduction band edge 51 and an energy of valence band edge 52. A wave function 53 of an electron (a quantized electron""s (hole""s) energy state 58 of a conduction band) and a wave function 54 of a hole (a quantized electron""s (hole""s) energy state 59 of a valence band), which contribute to emission, overlap in the physical relationship of a vertical direction only for extremely in part (a range is shown in the drawing by an arrow), and the distribution of electrons (holes) of the overlapped part (rise of a wave function to the upper part) is small. Accordingly, there is a problem that emission efficiency is exceedingly low.
On the other hand, the aforementioned another laser device structure in which the type-II superlattice constituted of the GaAsSb layer and the InGaAs layer which are thin as many as 1 to 10 molecular layers becomes the energy band structure shown in FIG. 10. In this case, because the InGaAs layer 55 and the GaAsSb layer 56 are exceedingly thin, subbands 62, 63 are formed, and the structure becomes the energy band structure just like a type-I quantum well used in a usual quantum well laser. As a result, high emission efficiency is obtained.
In this case, however, the subband 62 at the side of the conduction band is formed at the side of higher energy than the energy of conduction band edge of the InGaSb layer 55 and the subband 63 at the side of the energy of valence band is formed at the side of lower energy than the energy of valence band of the GaAsSb layer 56. Accordingly, there is a problem that an emission wavelength is shifted to a short wavelength. To enable realization of a long wavelength, the In composition of InGaAs and the Sb composition of the GaAsSb layer need be increased. Because the increase of these types of composition results in the increase of lattice strain, however, the lattice strain which exceeds 2.3% need be introduced to realize the emission wavelength of the 1.3 xcexcm band in this structure. In general, a device in which the lattice strain which exceeds 2% is introduced into an active layer may cause a problem in terms of life and reliability, and it is difficult to put the device to practical use.
An object of the present invention is to provide a semiconductor laser which has an active layer of a lattice strain of less than 2% on an average on a GaAs substrate and can be used in a long wavelength band of a 1.3 xcexcm band or more and a photo module which uses the semiconductor laser.
To attain this and other objects, a semiconductor laser device of the present invention has a first semiconductor layer and second semiconductor layers, both of the first layer and the second layers becoming an active layer on a semiconductor substrate, and makes the first semiconductor layer and the second semiconductor layers adjacent to each other and laminates them. The semiconductor laser device forms a type-II heterojunction structure in which an energy of conduction band edge of the first semiconductor layer is larger than an energy of conduction band edge of the second semiconductor layers and an energy of valence band edge of the first semiconductor layer is larger than an energy of valence band edge of the second semiconductor layers, and has third semiconductor layers on top and bottom of the active layer, of which the energy of conduction band edge is larger than the second semiconductor layers and the energy of valence band edge is smaller than the second semiconductor layers. In the semiconductor laser device, the second semiconductor layers are arranged respectively sandwiching the first semiconductor layer on top and bottom of the first semiconductor layer, and the thickness of the first semiconductor layer is a degree of thickness in which a wave function of an electron of a quantum well formed at the side of a conduction band by making the second semiconductor layers well layers is coupled, and the first semiconductor layer is made thinner than each of the second semiconductor layers.
FIG. 2 shows the energy band structure in the vicinity of the active layer of the aforementioned semiconductor laser. Here, as shown in the drawing by an arrow, because the growth direction of a multiple layer is from the left to the right as described previously, an expression of xe2x80x9con top and bottom of an active layerxe2x80x9d corresponds to the left and right of the active layer in FIG. 2 (in FIG. 1, and FIGS. 4, 8 described later, the expression corresponds to top and bottom as shown in the drawings). FIG. 2 shows from the left in order that a bottom third semiconductor layer 6a, a bottom semiconductor layer 4a, first semiconductor layer 5, a top second semiconductor layer 4b, and a top third semiconductor layer 6b are arranged. The first semiconductor layer 5 and the second semiconductor layers 4a, 4b become active layers and the third semiconductor layers 6a, 6b operate as barrier layers.
The thickness of the first semiconductor layer 5 is set in a degree of thickness in which a wave function of an electron of a quantum well is coupled, that is, the thickness in which the wave function of the electron can be coupled. Accordingly, in FIG. 2, by coupling the wave functions of two quantum wells at the side of the conduction band, a wave function 53 in which the electron is made to exist at the side of the conduction band is formed in a distribution area (a range shown by an arrow) of a wave function 54 of a hole, and the low emission efficiency which was the problem of the conventional type-II heterojunction structure is improved. The specific thickness of the first semiconductor layer 5 is, for example, approximately 6 nm or less according to the combination and composition of a material. This example differs from the related art in which both the aforementioned first and second semiconductor layers are made extremely thin as many as 1 to 10 molecular layers in that only the first semiconductor layer 5 is made thin in this manner.
Here, the reason why the first semiconductor layer 5 is made thin instead of the second semiconductor layers 4 is described simply. In the type-II heterojunction structure in which the size relationship of energy was defined as described above, the first semiconductor layer 5 forms a quantum well in a valence band and the second semiconductor layers 4 form quantum wells in a conduction band. In general, because the change of an energy state when the thickness of the quantum well is changed is smaller in holes than electrons, the valence band is difficult to influence by a quantum effect. As described previously, because the realization of the short wavelength of an emission wavelength when a quantum well is made thin causes a problem, in the present invention, a layer which makes the first semiconductor layer 5 thin was selected so that the quantum well can be formed at the side of the valence band with a low degree of the realization of the short wavelength even if the quantum well is made thin. Here, if the thickness of the first semiconductor layer 5 is 3 nm or more, the realization of the short wavelength of the emission wavelength when the quantum well is made thin is suppressed in such a degree that will not cause any problem.
As a combination of typical materials, the aforementioned semiconductor laser device is formed on a GaAs substrate, the aforementioned first semiconductor layer 5 uses GaAsSb and the aforementioned second semiconductor layers 4 use GaInAs or GaInNAs.
To enable realization of a long wavelength, the Sb composition or In composition need to increase, but both types of the composition result in an increase of lattice strain. Because the increase of the lattice strain gives a serious effect on the reliability of a device, the lattice strain (lattice strain of a thickness mean) as the entire active layer can be set to less than 2% by selecting a layer which increases the thinner first semiconductor layer 5 as the layer which increases the Sb or In composition for enabling the realization of the long wavelength. That is, the low lattice strain as the entire active layer can be realized by making the lattice-mismatch between the semiconductor substrate 1 and the first semiconductor layer 5 greater than the lattice-mismatch between the semiconductor substrate 1 and each of the second semiconductor layers 4.
Besides, it is easily understood that the present invention can apply to the realization of the long wavelength of the type-II laser on another type of substrate as well as that of the laser in which the type-II heterojunction structure on the GaAs substrate is used in the active layer. For example, the present invention is effective even for enabling the long wavelength of the type-II heterojunction structure using the GaAsSb/InGaAs formed on the InP substrate. By using this art, the long wavelength laser which can be realized only on the more expensive GaSb substrate than the INP can be realized with little lattice strain on the InP substrate.